1. Technical Field
This disclosure relates to semiconductor fabrication and more particularly, to a maskless process for forming diffusion contacts which are self-aligned and borderless to both gate conductors and isolation regions.
2. Description of the Related Art
To manufacture semiconductor memories, for example, dynamic random access memories (DRAM) as inexpensively as possible, it is important that innovative processes which reduce the total number of masks be sought. In the prior art, source-drain (bitline-node) diffusion contacts are provided which are borderless to an adjacent gate structure (e.g., including a wordline). The process for forming these borderless contacts with respect to the gate requires the application of a critical mask.
It should be noted that the so called "borderless" diffusion contact of conventional DRAM processes is borderless only to the adjacent gate and does overlap adjacent isolation regions. Misalignment of the contact mask and an active area mask, combined with contact overetch into adjacent isolation regions, may result in a deeper than desired junction forming along the sidewall of the active area silicon. A deeper junction degrades the short channel characteristics of a transistor (e.g., a metal oxide semiconductor field effect transistor (MOSFET)) formed in the active area silicon and increases the overall variation in threshold voltage of the transistor.
An additional concern of conventional DRAM processes is the effect of silicon defects induced in the memory array by implanted source-drain diffusions. Since these defects aggravate junction leakage mechanisms (bulk, surface and gate induced drain leakage (GIDL)), the dose of the implanted impurity (typically phosphorus or arsenic) is limited to rather small values (for example, &lt;1.times.10.sup.14 cm.sup.-2). The low doses used for the array source-drain implants contribute to increased total series resistance, which degrades performance.
Therefore, a need exists for a self-aligned maskless contact fabrication process which prevents misalignment and reduces junction degradation and leakage problems.